1. Field of the Invention
The present invention relates to a phase change memory, and particularly to a driving method and system for the phase change memory.
2. Description of the Related Art
With the growth in the use of portable electronic devices, the demand for non-volatile memory has increased. Among the various kinds of non-volatile memory, phase change memory is the most competitive next generation non-volatile memory due to its higher speed, lower power consumption, higher capacity, reliability, easier process integration and lower cost. By inputting specific current pulses, the material of the phase change memory can change quickly between two states. The phase change material changes its state only when receiving the specific current pulses and will not change its state when the input energy is discontinued. At present, Chalcogenide material is a popular and widely used phase change material used in modern phase change memory technology, wherein Ge2Sb2Te5 is the most popular. The material is also widely used in rewritable compact discs (CD-RW) and rewritable digital video discs (DVD-RW).
Phase change material in a phase change memory devices undergo at least two solid phases, a crystalline state and an amorphous state. Transformation between these two phases can be achieved by changing the temperature of the phase change material, wherein the temperature can be changed by specific current pulses or voltage pulses. The phase change material exhibits different electrical characteristics depending on its state. However, if the phase change material is incompletely crystallized (set), the resistance of the phase change material at the crystalline state is not sufficiently reduced, and therefore the range of the sensing margin is reduced. The general solution for the described problem is to increase the duration of the set current pulse or input a current pulse with high magnitude and short duration before the set operation. However, this uses more power and may decrease access speed of the phase change memory.
A novel phase change memory development technique, is to reduce the size of the phase change memory cell. When the size of the phase memory cell is reduced, the operating voltage of the phase change memory reduces and the conduction lines inside the phased change memory also becomes thinner. If the conduction line becomes thinner, the current carried in the conduction line also reduces. Furthermore, extra current pulse with high magnitude and short duration also increases the complexity of driving circuit design.